The present invention relates generally to sealing assemblies used in the oil and gas industry, and more particularly, to expandable downhole sealing assemblies in which an axial setting force is applied to radially compress and self-energize an expanding element such that stored radial forces in the expanding element effect and maintain a seal thereafter.
Downhole packers are well known in the oil and gas industry for sealing an annulus between various weight casings and mandrels at different underground locations in an oil well, for example, to exploit different production zones. Conventional downhole packers include multi-element elastomeric packers (see FIG. 7), packers including a single elastomeric element having metal or mesh back-ups (see FIG. 8), and inflatable packers. Elastomeric sealing elements are generally restricted for use in applications up to about 450° F., while high temperature and pressure applications require flexible or expanded graphite.
Packers having elastomeric sealing elements typically require a large axial setting force to deform the sealing element radially outward to reach the interior of the casing. Typically, the greater the setting force the more energy transferred radially outward. Once set, internal stresses within the sealing element work against the packer as lost energy, thus an axial maintenance force is required to resist the elastomeric memory of the element. As a result, the packer is not “self-energizing,” and will leak at a pressure higher than its equilibrium internal pressure without any sign of extrusion.
Geothermal packers used in high temperature applications (e.g., up to about 650° F.) can include braided graphite packing elements or compressed flexible graphite elements. These packers also require large axial setting forces to deform the element to reach the interior of the casing. One problem associated with the use of flexible graphite elements is that they are fragile, and thus susceptible to damage when the tool is lowered into the well. To protect the flexible graphite elements from damage during installation, the elements are typically covered with an elastomeric shell, which not only increases the complexity of the assembly, but also introduces the problems associated with elastomeric memory as discussed above.
Inflatable packers utilize air or other fluid injected into an expandable element to expand the outer shell to reach the interior of the casing. In inflatable packers, the inflation pressure must be higher than the system pressure in order to maintain a positive seal. Thus, like the other conventional packers discussed above, inflatable packers are not self-energizing. Additionally, any axial movement of the packer or the anchoring stop will relax the packer and reduce the sealing force accordingly. Further, when the temperature increases after setting, the element tends to expand, which further deforms the back-up system to create more gland volume or room. Further, changes in internal pressure as a result of thermal cycling affect the sealability.
Accordingly, what is needed is a self-energizing packer that overcomes the disadvantages of prior art assemblies in that the packer is structurally simple, requires a minimum setting force, obviates the need for axial maintenance forces, and is suitable for use with elastomeric, thermoplastic and graphitic elements, among other advantages.